Generally, the femtosecond laser pulse has good properties. Examples of the good properties include a short pulse time width, a high peaking power and a broad spectrum bandwidth.
Such characteristics enables a femtosecond laser system applied to various fields and examples of such various fields include ultraprecision of various materials, non-linear optics, biotechnology, chemistry, physics and health care application.
For example, a ultrashort laser pulse in a femtosecond range can minimize thermal diffusion in a processing range and generate no residual damage such that the femtosecond laser pulse can process a hard material difficult to mechanically process and a high peak power possessed thereby can realize a non-linear optical effect of multiphoton absorption used in processing a transparent material including glass and polymer into a variety of nano-scaled structures.
The laser stability means that such elements are maintained constant with respect to the time as a power applied to a target of a laser, a position of an applied beam, spatial traveling of a beam and a distribution type of beams.
In case a laser has to be applied to a target at a predetermined distance in laser processing, the laser stability is very important.
Especially, the laser stability is more important in case of ultraprecision laser processing having a nanometer leveled precision that uses a ultrashort laser such as a femtosecond laser.
The beam stability means that a beam is stably traveling at a uniform spatial position and a uniform angle and it is closely related with applications including laser processing.
The beam stability may be technically categorized into a beam positional stability related with changing positions from a target to a beam and a beam angular stability showing variations of angles when beams are focused on a target.
Typically, such the beam stability is called as ‘a beam pointing stability’.
Elements affecting the laser stability may include physical vibration, mechanical deformation, changes of thermal distribution, instability of a resonance capacity and internal and external factors such as air flow.
An ultrashort laser is sensitive to such internal and external factors out of them and various efforts are made to enhance the laser stability.
The ultrashort laser of which an exemplary example is a femtosecond laser is oscillated in an oscillator by mode locking and it is highly sensitive to micro-variations of an optical passage only to be sensitive to mechanical deformation of optical mounts generated by variations in ambient temperatures, such that power characteristics of the ultrashort laser could be sensitively changed.
Accordingly, most of femtosecond lasers are installed and operated in a clean room having a thermostatic chamber capable of maintaining an ambient temperature no more than ±0.5 stably for stable operation.
However, there have to be locally a difference between temperatures near optical mounts including mounts related with light pumping where a high power pumping light is applied and laser material mounts or near a laser power device having a cooling fan to emit much heat outside and a cooling device.
In other words, in case of a femtosecond laser, change in external temperatures of a space where a laser is installed and internal temperatures near optical components locally can affect power characteristics of laser.
Accordingly, it is vitally important to mechanically configurate a structure of a femtosecond laser system least sensitive to change in the temperatures.
An exemplary example of conventional femtosecond lasers is a laser using a Ti:Sapphire as a media.
A Ti:Sapphire material has a broad radiated spectrum band wavelength to 100 nanometers and can generate quite a short pulse even several femtosecond pulse.
Green light sources emitted from Dn:YVO4 pumped by a high power diode to pump energy outside are focused on Ti-Sapphire laser material in dozes to hundreds of micrometers.
At this time, the pumping light source is spaced apart several meters from Ti:Sapphire structurally and a power characteristic stability of the pumped lights is vitally important to operate the Ti:Sapphire stably.
For example, if a pointing stability of pumped lights is not proper, a spot position of the pumped lights focused in the Ti:Sapphire laser material, with a dozens of and hundreds of micrometer size is changing, failing to be uniform, such that the mode coupling between the pumped lights and the laser beam is changing constantly enough to deteriorate the laser power stability.
At this time, the mode coupling means the coupling configured to make the spatial distribution of laser beams coincide with the spatial distribution of pumped lights within the laser material substantially.
For example, when characteristics including an output power and an output beam direction are deteriorated, the result of the laser processed product cannot but be bad.
To overcome that, a reflection mirror is installed in an optical mount having a control device capable of controlling the beam direction precisely mounted therein and pumped lights are controlled to pass the reflection mirror and the output stability can be controlled accordingly.
However, the system has to be large-sized and complex to control relatively many optical components simultaneously and also the price of the system has to rise disadvantageously.
Accordingly, only one or two optical components are mounted in such a control device.
In contrast, the high power beam emitted from a semiconductor laser diode can overcome the disadvantage of the high-priced laser for light pumping, such that the disadvantages of the Ti-Sapphire laser including the price, size and stability of the equipment can be solved more easily.
Moreover, if the high power laser diode as the pumping light source can be positioned near to the laser material as close as dozens of or several centimeters, the stability of the ultrahigh speed laser can be enhanced more.
If an amplifier used so as to enhance the power of the ultrahigh laser can pump the lights by using the high power laser diode in a continuous wave mode not in a pulse mode, the amplifier can operate the laser more stably.
When a femtosecond pulse is generated first in a femtosecond oscillator in a mode locking, the pulse energy is very low by a nanojoule (nJ) and it is not proper to apply the femtosecond pulse to the laser processing.
To heighten the energy of the femtosecond pulse, Chirped Pulse Amplification (CPA) is used.
A pulse generated from a femtosecond oscillator is stretched longitudinally and timely. After that, the timely stretched pulse is applied to a femtosecond amplifier to amplify the pulse energy. Hence, the amplified pulse passes a pulse compressor to restitute a time width of the pulse to an initial femtosecond range. The pulses generated from the oscillator are employed as seeding pulses applied to the amplifier.
It is vitally important in the stability of the femtosecond amplifier to combine the seeding pulse and an amplifier resonance capacity mode with each other in the laser material stably.
Accordingly, the stability of the oscillator is more important in the femtosecond laser system having the amplifier.
According to one method for enhancing the stability of the femtosecond laser, an inner part of one aluminum block is digging to fabricate a laser case. Except a top cover, the other parts of the laser case is integrally fabricated as one body and mechanical deformation of the case generated by temperature change can be minimized.
In addition, there are efforts of minimizing the temperature change after a high power diode mounted module or a laser material module is fabricated of copper having a high heat conductivity, with cold water having a constant temperature flow therein.
Not only such modules but also a cooling water line enabling the cold water to flow there through is provided in the laser case to minimize the temperature change.
However, when the laser power is re-connected and operated the next day after operating the laser and power off, it is frequently found that laser characteristics are deteriorated.
This is because the mechanical deformation generated by the cooling during the power-off is not restored completely.
To overcome that, there are commercial femtosecond laser systems that encourage to operate the laser system including the pumping light source and the cooling device having the cold water for 24 hours to maintain a stable state.
However, it is not easy to maintain each of the optical mounts composing the laser resonance capacity at a set temperature.
To reduce power changes according to the time and spatial change of the beams having the pointing stability, it is vitally important to stably maintain the mode coupling of coupling the pumping lights applied from the outside with beams inside the resonance capacity within the laser material spatially.
It is not easy to maintain the mode coupling stably, because of the beam stability, if the pumping light applied outside is distant from the laser material.
Accordingly, the laser diode as the pumping light is located close to the laser material and the pumping light is directly applied to the laser material.
The high power pumping light is applied outside and the high power laser beam is generated in the laser resonance capacity. Because of that, the high power is transmitted to the optical mount having optical components such as an optical mirror, a laser platform and a laser as a type of heat.
The transmitted heat might cause the mechanical deformation of the optical mount and the mechanical deformation might change directions of the optical components slightly to make the arrangement of the laser resonance capacity in disorder, such that the laser power characteristics might be deteriorated.
Especially, the ultrashort laser such as the femtosecond laser uses mode locking so as to generate the femtosecond pulse and the mode locking is very sensitive to the deformation of the resonance capacity. Because of that, the stability of the femtosecond pulse could be deteriorated or the mode locking could be maintained any more to generate no femtosecond pulse.
For example, a type of the femtosecond laser realized by experiments is shown in an optical conceptual diagram of FIG. 1.
In FIG. 1, LC refers to a laser material and M1˜M6 refer to a reflection mirror. SAM refers to a saturable absorber mirror and DM refers to a dichroic reflection mirror. OC refers to a power coupling mirror and MD refers to a laser diode. WP refers to a half-wave plate and CL refers to a collimating lens and FL refers to a focusing lens.
A doted-line block shown in FIG. 1 refers to a light pumping unit (LPU).
To realize such an optical conceptual diagram, the optical components including those reflection mirrors are typically mounted in optical mounts, respectively, and they are coupled to laser platforms.
More specifically, FIGS. 2A and 2B show an apparatus realizing the doted-line block of FIG. 1. Using the high power laser diode outside the laser resonance capacity, energy is provided to the laser material positioned in the resonance capacity. In other words, FIG. 2A is a front view illustrating the light pumping units (LPU) having the conventional optical mounts coupled thereto independently and FIG. 2B is a plane view illustrating the light pumping units (LPU) having the conventional optical mounts coupled thereto independently.
The laser diode light pumping unit includes an optical fiber 110, an optical fiber mount 110a, a half-wave plate mount 110b, a collimating mount 110c, a focusing lens mount 110d, dichroic mirror mounts 110e and 110f, mount blocks 120a-120g installed in a laser platform 150 to support the mounts, respectively, and optical components corresponding to the others, respectively.
The result of FIG. 3 shows that the laser power is off in an optimized state and then on again after the laser power is completely cooled.
Here, even in case the power of the femtosecond laser in which the mode locking is stable is on, the mode locking is unbalanced and a continuous wave (CW) is generated. Accordingly, a mode locking starter has to be operated to be mode-locked again disadvantageously.